First You are doing your Batteries a great service by treating Them this way! I store at 60% Just remember to keep a check on them and dont let them get below the 40% mark (thats what I shoot for anyways}
Second Welcome to the forum From Bakersfield!
You have to manually copy the mission KMZ file from one DJI Fly installation to another.
The following instructions are for copying a KMZ file TO a DJI Fly installation. From these instructions you should be able to figure out how to export a...